[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] hvmloader: pass PCI MMIO layout to OVMF as an info table
On 01/11/21 16:21, Jan Beulich wrote: > On 11.01.2021 15:49, Laszlo Ersek wrote: >> On 01/11/21 15:00, Igor Druzhinin wrote: >>> On 11/01/2021 09:27, Jan Beulich wrote: >>>> On 11.01.2021 05:53, Igor Druzhinin wrote: >>>>> We faced a problem with passing through a PCI device with 64GB BAR to >>>>> UEFI guest. The BAR is expectedly programmed into 64-bit PCI aperture at >>>>> 64G address which pushes physical address space to 37 bits. OVMF uses >>>>> address width early in PEI phase to make DXE identity pages covering >>>>> the whole addressable space so it needs to know the last address it needs >>>>> to cover but at the same time not overdo the mappings. >>>>> >>>>> As there is seemingly no other way to pass or get this information in >>>>> OVMF at this early phase (ACPI is not yet available, PCI is not yet >>>>> enumerated, >>>>> xenstore is not yet initialized) - extend the info structure with a new >>>>> table. Since the structure was initially created to be extendable - >>>>> the change is backward compatible. >>>> >>>> How does UEFI handle the same situation on baremetal? I'd guess it is >>>> in even more trouble there, as it couldn't even read addresses from >>>> BARs, but would first need to assign them (or at least calculate >>>> their intended positions). >>> >>> Maybe Laszlo or Anthony could answer this question quickly while I'm >>> investigating? >> >> On the bare metal, the phys address width of the processor is known. > > From CPUID I suppose. > >> OVMF does the whole calculation in reverse because there's no way for it >> to know the physical address width of the physical (= host) CPU. >> "Overdoing" the mappings doesn't only waste resources, it breaks hard >> with EPT -- access to a GPA that is inexpressible with the phys address >> width of the host CPU (= not mappable successfully with the nested page >> tables) will behave super bad. I don't recall the exact symptoms, but it >> prevents booting the guest OS. >> >> This is why the most conservative 36-bit width is assumed by default. > > IOW you don't trust virtualized CPUID output? That's correct; it's not trustworthy / reliable. One of the discussions (of the many) is here: https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg04716.html Thanks Laszlo
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